The present invention relates to a method for the purification of raw caprolactam which contains as impurities primary amides according to general formula I ##STR2## wherein R is a hydrocarbon radical having from 1 to 14 carbon atoms, in particular hexahydrobenzamide, and other by-products. The present invention further refers to the caprolactam thus purified.
Methods for the purification of the raw caprolactam which contains amides as hereinbefore defined, in particular hexahydrobenzamide and other by-products, are known. Said methods may be grouped in two different classes, and in particular, they may be classed as methods having a chemical nature and methods having a physical nature. Thus e.g. a method having a chemical nature is claimed in the Japanese patent application No. 46-23751, said method being characterized by the fact that raw caprolactam is treated with a sodium hypochlorite solution. Another method having a chemical nature is claimed in DOS No. 1.926.932 and consists in treating the molten raw caprolactam with metal oxides. The methods having a physical nature mainly consist in distilling and/or rectifying the caprolactam. Thus e.g. in English Pat. No. 1,157,416 a method for the purification of caprolactam by distillation is described. In Italian patent application No. 20612 A/79 of the Applicant a method for the purification of raw caprolactam by a combination of distillation and rectification operations is described and claimed.
This last method described in patent application No. 20612 A/79 of the Applicant, while it permits one to obtain the greater part of the caprolactam in such a pure form that it may be employed industrially to make polycaprolactam, has however the drawback that rectification or distillation tails, which contain, besides one or more amides as hereinbefore defined and other by-products, also considerable amounts of caprolactam must be discarded. In order to render the process economical it is necessary to recover said caprolactam from the rectification tails.